The present invention relates to a pulse width modulation amplifier circuit having a balanced transformer-less (BTL) output connection and particularly to a filter circuit for eliminating a carrier signal.
Pulse width modulation amplifiers (referred to as PWM amplifiers) are known for modulating a high frequency triangular-wave carrier signal with an analog signal such as an audio signal into a pulse width modulation, signal Such amplifiers power-amplify the pulse width signal and eliminate the carrier signal by means of a demodulation filter. Those amplifiers are very efficient for power amplification and thus they have recently been used in audio devices intended for automobiles.
In FIG. 3, an example of PWM amplifiers is shown. An analog signal at an input terminal 1 is applied to the inverted input terminal of a comparator 2, while its non-inverted input terminal has applied thereto an output signal from a high frequency (e.g., about 200 KHz) triangular-Wave generator 3. This causes the carrier signal to be modulated into a pulse width signal. After being amplified by a driving amplifier 4, the pulse width modulation signal from the comparator 2 is amplified by a pulse amplifier (power amplifier) 5 consisting of power FET's of the N-channel MOS type. The carrier signal is eliminated by a filter circuit consisting of a choke coil 6 and a condenser 7, and the remaining signal drives a load, for example a speaker 9, connected with the output terminals 8.
FIG. 4 shows a conventional BTL circuit where two such PWM amplifiers as described above are used.
In the BTL circuit, the analog signal applied to the input terminal 1 is converted into first and second analog signals which differ in phase from each other by 180 degrees by means of a differential circuit 10 or the like. The first analog signal is converted into a pulse width modulation signal by the first comparator 2, is amplified by a driving amplifier 4 and a pulse width amplifier 5 has its carrier eliminated by a choke coil 6 and a condenser 7, and is led to an output terminal 8. In the same manner the second analog signal is converted into a pulse width modulation signal by a second comparator 2', is amplified by a driving amplifier 4' and a pulse width amplifier 5', has its carrier eliminated by a choke coil 6' and a condenser 7', and is led to an output terminal 8'.
Both ends of a load, e.g., a speaker or the like, are connected between the above output terminals 8 and 8', therefore the load 9 is supplied with demodulated analog outputs of opposite phase by a pair of PWM amplifiers. Since the voltage between the terminals of the load becomes twice the output voltage that can be gained from a single PWM amplifier, it is logically possible to have the output apply to the load 9 four times the electric power that can be obtained with a single PWM amplifier.
In the BTL-PWM amplifier mentioned above the first and second PWM amplifiers need to be provided with carrier eliminating filters each consisting of a choke coil and a condenser. Since the choke coils are supplied with pulse signals such as those of about 200 KHz, toroidal chock coils are usually used in order to prevent the occurrence of radiating noise.
Large currents occur in each of the above-mentioned chock coils as a result of being driven by audio outputs, and this causes the choke coils to be saturated by DC magnetization, which untowardly lowers the function of the filter. To prevent this problem, the size of the core of each filter may be made large.
Furthermore, it is difficult to determine the cutoff frequency of the filter circuit consisting of a choke coil and a condenser because of two conflicting requirements. First the cutoff frequency should be set considerably high in order not to affect the output in the audio signal band, and second, the cutoff frequency should be set considerably low in order to attain sufficient attenuation of the carrier.